A lens is a device that can focus (converge or diverge) one or more wavelengths of light. The term light is understood to include both visible electromagnetic radiation, and other wavelengths of electromagnetic radiation.
A variable (or adjustable) lens is a lens in which one or more properties of the lens can be controllably adjusted e.g. in which either the focal length or the position of the lens can be altered.
DE 19710668 describes a variable lens system 40, as illustrated in FIGS. 1A and 1B. The lens system 40 comprises a resilient membrane 42 filled with a fluid 44. The pressure of the fluid 44 within the membrane is controlled by means of a pump 46. The dotted line illustrates the optical axis 90 of the lens system 40. The membrane 42 acts as a variable lens, with the shape (and hence power) of the lens varying in dependence upon the pressure of the fluid 44. FIG. 1A shows the fluid 44 at a low pressure i.e. with the membrane 42 forming a biconcave lens. FIG. 1B shows the fluid 44 at a higher pressure, with the membrane 42 forming a bi-convex lens.
Such a system posses a number of disadvantages. Due to the movement of the membrane surface, it is difficult to maintain good optical properties and it is susceptible to vibrations. Further, it is susceptible to mechanical fatigue. Control of the shape of the lens is not only dependent upon both gravity and the pressure of the fluid 44, but also the resilience of the membrane 42. Consequently, obtaining a range of desired lens shapes can be problematic, particularly if the elasticity of the membrane 42 changes over time. Additionally, flexible membranes are not normally gas tight, resulting in the evaporation of the fluid from the device over time.
Variable focus lenses based on electrowetting devices are also known. Electrowetting devices are devices that utilise the electrowetting phenomenon to operate. In electrowetting, the three-phase contact angle is changed with applied voltage. The three-phases constitute two fluids and a solid.
International patent application WO 99/18456 describes a variable focus lens utilising the electrowetting effect. FIG. 2 is a cross-sectional view of such a typical optical device. The device has two immiscible fluids 80, 87 confined in a sealed space 92 (i.e. a chamber or cavity). The term immiscible indicates that the two fluids do not mix. The first fluid 80 is an insulator (e.g. silicone oil) and the second fluid 87 electro conductive (e.g. a mixture of water and ethyl alcohol). The first fluid 80 and the second fluid 87 have different refractive indices.
A voltage from a voltage supply 50 can be applied to the two electrodes 51, 52 so as to produce an electric field between the first fluid 87 and the electrode 52 (an insulating layer 65 prevents the second electrode 52 contacting the conductive second fluid).
By varying the voltage applied to the second fluid 87, the shape of an interface 85 between the first fluid 80 and the second fluid 87 is altered, so as to change the lens function provided by the interface 85. The device in FIG. 2 has a water-repellent film 60 of diameter D1 on the insulating layer 65 surrounded by a ring of a hydrophilic agent 70 so as to locate the first fluid 80.
This electrowetting lens also possesses a number of disadvantages. For instance, the lens shape is defined by a variable voltage, with the lens shape being affected by any non-uniformities of the insulating layer. The configuration requires a relatively high voltage to alter the shape of the interface 85. Further, the insulating layer may suffer from charging (especially at high voltages). If the insulating layer is charged unequally, this will lead to unequal contact angles, and thus to non-spherical lens. Further, it is not possible for such a lens to vary from being a positive power lens to a negative power lens (or vice versa).
It is an aim of embodiments of the present invention to provide a variable lens that addresses one or more problems of the prior art, whether referred to herein or otherwise. It is also an aim of the present invention to provide optical devices incorporating such lenses, and methods of manufacturing such lens and such devices.